Forensic Linguistics is the application of linguistics to legal issues. That is a starting point, but like all answers it is imperfect and serves only to stimulate more questions. For example, what does ‘the application of linguistics’ mean?
When Forensic Linguistics is referred to as an application of linguistics or, more concisely, an applied linguistic science, the word applied is not necessarily being used in the same sense as, for example, in the phrase applied statistics, where what is being applied is a theory underpinning a particular science to the practice of that science. Forensic Linguistics is, rather, the application of linguistic knowledge to a particular social setting, namely the legal forum (from which the word forensic is derived). In its broadest sense we may say that Forensic Linguistics is the interface between language, crime and law, where law includes law enforcement, judicial matters, legislation, disputes or proceedings in law, and even disputes which only potentially involve some infraction of the law or some necessity to seek a legal remedy. Given the centrality of the use of language to life in general and the law in particular, it is perhaps somewhat surprising that Forensic Linguistics is a relative newcomer to the arena, whereas other disciplines, such as fingerprint identification and shoeprint analysis, are much older, having a well‑established presence in judicial processes.
The application of linguistic methods to legal questions is only one sense in which Forensic Linguistics is an application of a science, in that various linguistic theories may be applied to the analysis of the language samples in an inquiry. Thus, the forensic linguist may quote observations from research undertaken in fields as diverse as language and memory studies, Conversation Analysis, Discourse Analysis, theory of grammar, Cognitive Linguistics, Speech Act Theory, etc. The reason for this reliance on a broad spectrum of linguistic fields is understandable: the data the linguist receives for analysis may require that something is said about how the average person remembers language, how conversations are constructed, the kinds of moves speakers or writers make in the course of a conversation or a written text, or they may need to explain to a court some aspects of phrase or sentence structure.